There is some debate over whether he killed himself or was killed by his fellow Khmer Rouge while in custody. He never made it to trial. Several of his comrades did go to trial and were convicted of crimes against humanity. The Khmer Rouge were recognized by the rest of the world (most of the western world including the US) because they were in conflict with the communist Vietnamese government. It was the Vietnamese who liberated Cambodia and chased out the Khmer Rouge, setting up a new government. Cleary the US could not support a government backed by the Vietnamese, so they continued to endorse Pol Pit and the Khmer Rouge. The rest of the world followed suit and the Khmer Rouge were allowed a seat at the UN for many years after they were deposed from power. An enemy of my enemy is my friend.

Pol Pot was supported because he was a US/UK/China backed proxy against a fearful but unfounded idea that Vietnam wanted to conquer and turn Vietnam/Laos/Cambodia/Thailand into it's own version of a united Socialist bloc.

Khmer Rouge in power had already purged a significant majority of the more "Orthodox" Marxist Leninists in the party in favor of Pol Pot's rabid Nationalist line. By the time Pol Pot lost power he and the Khmer Rouge had fully and completely abandoned Marxism, openly praising the USA, and Ieng Sary was even praising Ronald Reagan.

Source: Cambodia 1975 - 1982, Michael Vickery.

Here's a post I made else where on Cambodia that may provide some insight.

Pol Pot's revolution was an entirely peasant based uprising. Democratic Kampuchea was an attempt to turn the entirety of Cambodia into a peasant based agrarian society. The Cambodian peasantry didn't know private property. All land and property belonged to the French, the Aristocracy, and the Japanese. The Cambodian peasantry didn't know education. The French purposefully kept literacy and education rates as low as they could to prevent political unrest. Once the Japanese were run out of Cambodia, a new Aristocracy took control and through Prince Shinouk's attempts at neutrality Cambodia began to be infiltrated with Vietnamese. Cambodian peasant relations with the Vietnamese had always been sour as the Vietnamese were better educated and enjoyed higher privileges in Cambodian society than even Cambodian peasants. Eventually a military dictatorship came to power, one that allied with the United States and was complacent in the death of hundreds of thousands of Cambodian innocents. The peasants who had never earned much money were packed into cities to escape bombing runs, left with no money to buy food or provisions, left to starve.

According to the Khmer itself, the US bombing raid was the final rallying cry for the Cambodian peasantry, which fully allied itself with the Angkar, the Khmer Rouge before they fully adopted Marxist/Communist posturing. The Khmer state reflected what the Cambodian peasantry knew: It was an agricultural state, one that favored agricultural labor over education, one that favored full communalization of property, one that favored Cambodian ethnicity, one that favored traditional Cambodian peasant life. Property, education, money, multi culturalism were all symptoms of things that had kept the Cambodian peasants poor and destitute, and in the new Cambodian state, those traits would be eradicated.

Pol Pot was overwhelmingly popular with the Cambodian peasantry. Enough to empty entire cities through force. You know what happened to the Cambodian people afterwards and Pol Pot eventually began to alienate the peasantry through over work and dehumanization and using starvation as a weapon and means of control ("If someone is very hungry, the Angkar will take him where he will be stuffed with food."). When the Khmer Rouge were removed from power through collaborators with the Vietnamese a sizable portion of the Khmer Rouge forces went with Pol Pot into exile. These forces were the ones that came from the provinces within Cambodia that saw the least amount of strife, as certain districts did not suffer nearly as immensely as others. They were also propped up with US and UK funding but their entire goal was to remove Vietnam from Cambodian soil. In interviews post downfall Pol Pot was steadfast in believing that what he was doing was right, and he still had support from a faction of the peasantry. While I've never heard that particular saying in your question, Pol Pot's isolation from the absolute worst situations in the Khmer Rouge, his belief that all that oppose the Angkar were not true Cambodian peasants, and his continued support in exile gave him all the vindication that he needed. Sources:

The Khmer Rouge: Ideology, Militarism, and the Revolution That Consumed a Generation

Pol Pot's Little Red Book

Brother Number One: A Political Biography Of Pol Pot

Pol Pot Plans the Future

Nate Thayer's AMA and the reference interview as well.

According to Nicholas Wheeler's Saving Strangers, this is partly due to the fact that most countries believed in pluralism until the 1990's. Pluralists stress the importance of sovereignty over anything else, and so humanitarian interventions were never really a thing. In fact, when Vietnam invaded, they were condemned for trying to set up a puppet government. Vietnam didn't even use humanitarianism as an explanation for the invasion, but rather the fact that the Khmer Rouge attacked them first. Due to pluralism, countries that were fully aware of what Pol Pot did not think they had the right to invade and still recognized his government over the new Vietnam-back government. Therefore, they would never have tried him for war crimes at the time. By the 1990's, the international system had become open to the idea of humanitarian intervention because pluralism was replaced by solidarism. The Khmer Rouge agreed to turn him over to an international tribunal in the 90s, but he died shortly after.

Not sure if this will get seen, but I've actually done a lot of research on post-war Cambodian history and can speak on the subject

Officially, the reason is that Prime Minister Hun Sen delayed holding tribunals as the country was not "stable" enough for 35 years. To understand this though, you have to be cognizant of the political environment and the political priorities dictated by neoliberalization, in which stability was paramount.

The United Nations Transitional Authority in Cambodia that was mandated to oversee the peace process and formation of an independent Cambodian state faced an extremely fractured Cambodia where the Khmer Rouge still possessed substantial political power well into 90s, as they continued to wage a civil war from the Western provinces. This was partially due to SAS training and Western humanitarian aid that was essentially funneled to Khmer Rouge fighters (a fact which John Pilger explores in his documentaries). A major objective of the international community involved in UNTAC was creating stability necessary for the transition of the market economy, as until the fall of the Soviet Union, the country had survived almost entirely on USSR development aid, and the years immediately leading up to the Peace Process had seen a substantial demand for neoliberal reform (especially in regards to FDI and land tenure reform) by the Cambodian People's Party (lead by Hun Sen who was himself a former Khmer Rouge commander) and the international community. This entire period was profoundly unstable- although the UN elections had selected King Sihanouk's successor by a landslide, when Hun Sen threatened to launch a succession movement, the international community effectively let him be 'co-president', and later more or less turned a blind eye when he launched a coup from the position and consolidated power.

The emphasis on unifying Cambodia under Hun Sen in the face of such a political rift produced a peace treaty that was extremely concessionary to the Khmer Rouge. By the end of the peace process in the late 90s, near the end of Pol Pot's life, they had been essentially fully reintegrated into the Cambodian government and military, even at the highest levels, and the Khmer Rouge were granted arguably the most mineral rich and valuable province in Cambodia to control. As a consequence, many of the former Khmer Rouge became the some of the most politically powerful members of the new state, which compounded their power as the wealth resulting from the market economy disproprtionately ended up in the hands of the political elite of an increasingly militarized authoritarian state (the proportion of military spending actually increased dramatically after "peace was declared). Even today, the ruling Cambodian People's Party is full of ex-Khmer Rouge. As a consequence, it wasn't very politically feasible to start tribunals that would threaten them and potentially threaten the economic stability of the country.

There have been a lot of answers addressing your second question, but not so many addressing the first, so I'll take a shot at it. While /u/rucheleh06 is right that Pol Pot died ages ago, there is still the very valid question of why he wasn't prosecuted. Quite simply, there wasn't an avenue for it. While today we have the International Criminal Court that deals with genocidal people and those who commit crimes against humanity, it's a fairly recent institution, having mostly been established in the wake of Yugoslavia and Rwanda. Calls for the prosecution of the Khmer Rouge did not start to take place until after the establishment of the court.

And why are we still not talking about the US backed bombings that happened during khmer republic govt trying to stop CPK's accent? I mean there were sizeable casualties there too.

Anyone know if the committee actually conducted a poll of the desires of the locals? Or did it all come down to lobbying and friends in high places?